Simultaneous switching transistors in densely packed circuits, such as in very large scale integrated (VLSI) circuits, cause current spikes in the power supply lines, which in turn cause voltage noise due to iR and Ldi/dt voltages. Non-invasive off-chip measurements or detection of power supply noise is impractical due to loading effects and tester constraints. However, information about supply fluctuations can be extremely useful for purposes of testing, as well as system reliability improvement.
Some power supply noise detection techniques have been described in H. Aoki et al., “On-chip Voltage Noise Monitor for Measuring Voltage Bounce in Power Supply Lines Using a Digital Tester,” Proceedings of the 2000 International Conference on Microelectronic Test Structures, pp. 112-117, 2000; and in A. Muhtaroglu et al., “On-die Droop Detector for Analog Sensing of Power Supply Noise,” IEEE Journal of Solid-State Circuits, Vol. 39, Issue 4, pp. 651-660, April 2004, the disclosures of which are incorporated by reference herein.
The technique described in H. Aoki et al. employs a comparator that compares the noisy supply to an external reference voltage. The comparator requires four clocks and the performance of the comparator strongly depends on the time constant of the capacitors in the design. However, the capacitors have to be sized such that the drain-to-gate capacitance of the transistors does not corrupt the measured data. Hence, this technique is extremely sensitive to sizing and does not have any calibration features to offset process and temperature variations. It also does not have any features for generating different reference voltages.
The technique described in A. Muhtaroglu et al. uses voltage sensors connected to the supply lines. It uses a complicated procedure to generate different voltage references. The reference generator requires two 32-bit DACs (digital-to-analog converter) to generate current references. Also, each sensor requires a dedicated current reference, since the calibration features are a function of the current reference (two 32 bit DACs). Also, this approach requires two separate sensors to detect overshoots and undershoots. It also requires two current references to set different thresholds for overshoots and undershoots.